The Ras signal transduction pathway responds to various extracellular signals, for example, growth factors, cytokines and an extracellular matrix (ECM) through the cell-surface receptors to play an important role in regulation of proliferation, differentiation and transformation of cells.
The activation of the Ras protein in normal cells begins by the interaction of such extracellular signals as growth factors with the cell-surface receptors, and then the activated Ras protein interacts with Raf, a serine-threonine protein kinase, to activate Raf (see Non-patent Document 1 and Non-patent Document 2). It is known that with Raf, there are three types of isoforms of A-Raf of 68 Kd, B-Raf of 95 Kd and Raf-1 (c-Raf) of 74 Kd, and each is different in the aspects of the interaction with the Ras protein, the capacity of activating the substrate MEK, the expression and distribution in organs and the like, and the study with the use of a knockout mouse shows that all three A-Raf, B-Raf and Raf-1 are essential in survival. The activated Raf successively activates the substrate MEK by phosphorylation and the activated MEK activates ERK 1 and ERK 2 (MAPK). The activated ERK finally activates various substrates such as transcription factors in the cell nucleus and cytoplasma to bring about cellular changes (proliferation, differentiation and transformation) in response to the extracellular signals. These cellular changes including proliferation in normal cells are appropriately regulated but it is observed that in human cancer cells, about 20% of the Ras protein is mutated to be always in an activated state (GTP complex) and it is known that as a result, the growth signal to the Raf/MEK/ERK cascade is maintained to play an important role in the growth of human cancer cells (see Non-patent Document 3). Further, in the recent study, it is reported that the mutation of B-Raf is confirmed in 66% of melanomas, 15% of colon cancers and 14% of liver cancers, and the Raf/MEK/ERK cascade is in an activated state (see Non-patent Document 4).
In addition to the role as a direct downstream effector of the Ras protein in the Raf/MEK/ERK cascade as described above, the Raf kinase is known to play a key role in controlling the apoptosis of cells by various mechanisms (see Non-patent Document 5).
Thus, the techniques of blocking the Ras signal transduction pathway which plays an important role in the proliferation of cancer cells by inhibiting the Raf kinase as a target can be thought useful. Actually, it is reported that by inhibiting the expression of Raf with the RNA antisense, the growth of various human cancers is inhibited in vitro and in vivo (see Non-patent Document 6).
Cancer cells take in oxygen and nutrients necessary for survival and growth from the surrounding environment. In a solid tumor, these substances are supplied by simple diffusion until the solid cancer reaches a certain size. However, as the solid tumor grows to form a region 1 to 2 mm or more apart from the nearest blood vessel, this region forms a hypoxia region where the oxygen concentration is low, the nutrients are poor and the pH is low. Against to these stresses, tumor cells respond by various angiogenesis factors to stimulate the formation of a new blood vessel from the neighboring vascular endothelial cells. The angiogenesis thus started is thought to be essential in the growth of the solid tumors. There are a number of reports which suggests the relationship between VEGF (vascular endothelial growth factor), which is a growth factor specific for the vascular endothelial cells, and cancers, and the drugs which target VEGF or the tyrosine kinase activity of its receptors have recently been developed (see Non-patent Document 7 and Non-patent Document 8). Up to now, it is known that VEGF bonds to three types of receptor tyrosine kinases i.e. VEGFR-1 (flt-1), VEGFR-2 (KDR) and VEGF-3 (Flt-4), and since KDR performs strongly ligand-dependent autophosphorylation, KDR is thought to be essential to VEGF-dependent biological responses including angiogenesis.
On the other hand, a number of factors, which are involved in angiogenesis, are known in addition to VEGF and they act on vascular endothelial cells that play a key role in angiogenesis. The development of inhibitors of proliferation and function of vascular the endothelial cells that specifically act on vascular endothelial cells is strongly desired as therapeutic agents for angiogenic diseases such as cancers.
With respect to the relationship between the two cancer treatment targets, i.e. Raf and angiogenesis, an interesting report has recently been made. The activation of B-Raf and Raf-1 depends on not only the Ras protein but also growth factor signals. Basic fibroblast growth factor (b-FGF) activates Raf-1 through PAK-1 (p21-activated protein kinase-1) by the phosphorylation of serine 338 and 339 of Raf-1 to protect endothelial cells from apoptosis non-dependently to MEK 1. The VEGF signal activates Raf-1 through Src kinase by phosphorylation of tyrosine 340 and 341 of Raf-1 to protect endothelial cells dependently to MEK 1. By this report, it has been clarified that Raf plays a key role in not only the growth of cancer cells but also the control of survival of endothelial cell on angiogenesis (see Non-patent Document 9).
Further, angiogenesis is a physiological phenomenon essential in embryonic formation of the fetal period, wound healing of an adult, the menstrual period of an adult female and the like but it is reported that abnormality of angiogenesis in an adult individual relates to psoriasis, atherosclerosis, chronic rheumatoid arthritis and diabetic diseases (see Non-patent Document 10 and Non-patent Document 11), and inhibition of angiogenesis is useful for treating these diseases with the abnormality of angiogenesis.
Heretofore, a number of urea compounds which exhibit anticancer action by inhibiting any of Raf and kinases relating to angiogenesis (see Patent Documents 1 to 13). However, these compounds have a problem of solubility in water due to the high hydrophobicity and high crystallinity attributed to the phenylurea skeleton. Particularly in the case of oral drugs, the property of inferior solubility in water tents to lead to severe problems in clinical development such as poor bioavailability, unstable efficacy due to the individual difference in PK among patients or tendency of accumulation (see Non-patent Document 11 and Non-patent 13). For example, it is reported that the following compound Bay 43-9006 (Patent Document 5, Example 41):
is a Raf-1 and B-RAF inhibitor and is also an inhibitor of kinases relating to the angiogenesis and the progression of a cancer including KDR, VEGFR-3, Flt-3, c-KIT and PDGFR-β (see Non-patent Document 14). However, the results of the phase I clinical study of the compound are reported (see Non-patent Document 15) and the compound is pointed out to have problems of high interpatient PK variability, tendency of accumulation upon multiple dosing and the like, which are due to high lipophilicity and low water solubility.    Patent Document 1: International Publication No. 98/52559 Pamphlet    Patent Document 2: International Publication No. 99/32106 Pamphlet    Patent Document 3: International Publication No. 99/32436 Pamphlet    Patent Document 4: International Publication No. 99/32455 Pamphlet    Patent Document 5: International Publication No. 00/42012 Pamphlet    Patent Document 6: International Publication No. 02/62763 Pamphlet    Patent Document 7: International Publication No. 02/85857 Pamphlet    Patent Document 8: International Publication No. 03/47579 Pamphlet    Patent Document 9: International Publication No. 03/68223 Pamphlet    Patent Document 10: International Publication No. 03/40228 Pamphlet    Patent Document 11: International Publication No. 03/40229 Pamphlet    Patent Document 12: International Publication No. 03/68746 Pamphlet    Patent Document 13: International Publication No. 03/80064 Pamphlet    Non-patent Document 1: Trends Biochem. Sci., Vol. 19, 474-480, 1994    Non-patent Document 2: Science, Vol. 264, 1463-1467, 1994    Non-patent Document 3: Annual Reports in Medicinal Chemistry, Vol. 29, 165-174, 1994    Non-patent Document 4: Nature, Vol. 417, 949, 2002    Non-patent Document 5: Biochemical Pharmacology, Vol. 66, 1341-1345, 2003    Non-patent Document 6: Nature, Vol. 349, 426-429, 1991    Non-patent Document 7: J. Clinical Oncology, Vol. 21, 60-65,    Non-patent Document 8: Expert Opinion Investigational Drugs, Vol. 12, 51-64, 2003,    Non-patent Document 9: Science, Vol. 301, 94-96, 2003    Non-patent Document 10: New England Journal of Medicine, Vol. 333(26), 1757-63, 1995    Non-patent Document 11: Angiogenesis, Vol. 5(4), 237-256,    Non-patent Document 12: Pharmazeutische Industrie, Vol. 64(8), 800-807, 2002    Non-patent Document 13: Pharmazeutische Industrie Vol. 64(9), 985-991, 2002    Non-patent Document 14: AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics, Proceedings, p. 69, No. A78    Non-patent Document 15: American Society of Clinical Oncology, Annual Meeting (May 18 to May 21, 2002) Abstracts, Nos. 121, 1816 and 1916, 2002.